The 17-year-old boy was knifed on the bus in Cricklewood, north west London, at about 5.30pm yesterday.
The teenager was stabbed six times, according to unconfirmed reports. One witness claimed the teenager was chased by two men in helmets.
The witness added that one of the attackers was “wielding a long knife”, Mail Online reported.
Emergency services raced to the scene and the youngster was taken to hospital.
In a statement yesterday, a Met Police spokesman said: “Police were called at approximately 5.30pm on Saturday April 27 to reports of a stabbing on a bus in Cricklewood Lane, near the junction with Cricklewood Broadway, NW2.
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Officers and London Ambulance Service attended and found a male, believed aged 17, suffering stab injuries.
He has been taken to a central London hospital for treatment condition awaits.
Enquiries into the circumstances are ongoing. No arrests have been made and a crime scene remains in place at the scene.
They added that there have not yet been any arrests in connection with the stabbing.
Anyone with information should contact police on 101 and quote CAD 5859/27Apr or call Crimestoppers anonymously on 0800 555 111.